GROWTH AND VITALITY OF EPIPHYTIC LICHENS .2. MODELING OF CARBON GAIN USING FIELD AND LABORATORY DATA

Photosynthetic and respiratory CO2 gas exchange was measured under con trolled climate conditions in the laboratory in two epiphytic lichens, Lobaria pulmonaria and Platismatia glauca, with the aim of modelling their net productivity using field microclimate data. For both, the th allus water content (WC) and the light intensity had the greatest impa ct on photosynthesis. L. pulmonaria had optimum net photosynthesis (NP ) at WCs between 75-175% of the thallus dry weight (DW), while P. glau ca required a WC of c. 85% for maximal NP without depression at higher WCs. Both species reached light compensation of NP at 5-10 mu mol pho tons m(-2) s(-1) and were saturated at 100-150 mu mol photons m(-2) s( -1). Respiratory CO2 loss corresponded to 35-40% of gross photosynthes is at. 85-100% WC and 15 degrees C, in both species. Growth of the two species were followed in transplanted thalli during a 16-month period at two contrasting sites, a forest edge adjacent to a 15 year old cle ar-cut and within the interior of a mature Picea abies forest. At thes e sites, the microclimate parameters; light, temperature, relative hum idity (RH) and thallus WC were also monitored. Judged from the microcl imate data, the lichens were active for 13-19% of the time with thallu s WC monitoring, where >60% of the active time occurred in darkness. W hen photosynthetically active, the edge transplants received a 2-3 tim es higher light dose and were active for a longer accumulated time com pared to the interior transplants. The field microclimate data in conj unction with the laboratory data predicted a 4 times higher DW yield o f the edge transplants compared to the interior transplants. However, the DW yield of L. pulmonaria was overestimated at the edge and undere stimated for P. glauca in the interior by our model. Possible reasons for these discrepancies and the validity of using laboratory data and microclimate monitoring to predict growth rates of lichens under valid ity field conditions are discussed.